Development of a noncontact crack detection technique using pulse laser with high repetition rate

Abstract

This thesis presents the development of a crack detection method in silicon wafer using high repetition rate pulse laser. Conventional non-contact laser ultrasonic inspection method used pulse laser, which can only generate wideband ultrasonic waves in frequency domain but not narrowband ones. In this study, Narrowband ultrasonic signals are generated in frequency domain through high repetition rate pulse laser, and it is used to detect cracks in silicon wafer. When we excite low frequency (LF) and high frequency (HF) inputs to a silicon wafer, the presence of nonlinear sources, such as cracks, creates nonlinear ultrasonic modulation components at spectral sideband. Two narrowband ultrasonic waves with two distinctive central frequencies are generated by high repetition rate pulse laser to investigate the crack induced nonlinear ultrasonic modulation, and they are used to indicate the existence of the crack. Moreover, a probability of detection (POD) is established for the proposed crack detection technique.